Riding lawn mowers and similar vehicles are typically driven by hydrostatic transmissions. These known transmissions typically include a hydraulic pump. The input shaft of the pump is coupled to an internal combustion engine so that the pump works to deliver fluid to a hydraulic motor. The output shaft of the motor is coupled to a vehicle wheel.
Any shock load to the hydraulic motor, e.g. in the form of an impediment to the rotation of the wheels, can impart a shock load to the entire hydraulic transmission. This shock load typically in the form of excessive pressure, can negatively affect the components to the hydrostatic transmission. Many known designs of hydraulic transmissions do not integrate components, such as relief valves or shock valves, to protect the pump and/or the hydraulic motor, as well as the other components of the hydraulic system.
Some known system designs that have incorporated relief valves into the hydraulic transmission have positioned the relief valve within the pump. Such a design protects the pump from shock loads but typically does not protect the other components, e.g. the hydraulic motor, that first experiences the shock. Specifically, if the output shaft of the hydraulic motor experiences any sudden resistance, then a surge of pressure initially will travel through the motor possibly causing damage. The pressure spike may also damage other components between the motor and pump prior to being dissipated within the pump.
Other known system designs include two one-way relief valves disposed in the fluid path in the hydraulic motor. A first relief valve allows pressure to flow in one direction in response to a pressure spike and a second relief valve allows pressure to flow in the opposite direction in response to a pressure spike. This system requires the use of two valves. It also does not allow for adjustment of the load at which the valve will allow fluid to pass, other than exchanging the biasing portion of the valve with a different biasing portion.